Hard Disks with rotating magnetic platters are being replaced with more reliable Solid-State Drive (SSD) using semiconductor flash memory. NAND flash memory uses electrically-erasable programmable read-only memory (EEPROM) cells that store charge on a floating gate. Cells are typically programmed by an avalanche current, and then erased using quantum mechanical tunneling through a thin oxide. Unfortunately, some electrons may be trapped in the thin oxide during program or erase. These trapped electrons reduce the charge stored in the cell on subsequent program cycles, assuming a constant programming voltage. Often the programming voltage is raised to compensate for trapped electrons.
As the density and size of flash memory has increased, the cell size and its reliability and lifetime have all been reduced. The number of program-erase (P/E) cycles that a single-level flash memory is guaranteed to be able to withstand is about 100,000 cycles, which allows for a lengthy lifetime under normal read-write conditions. However, the smaller flash cells have experienced a disturbingly higher wear. Newer two-level cell flash memories may have an endurance of less than 10,000 P/E cycles, and Triple-Level Cells (TLC) may have an endurance of between about 500 to about 1,500 P/E cycles. If current trends continue, future flash memories may only allow for 300 program-erase cycles. Such a low endurance could severely limit implementations of flash memory, and applications for SSD. A high-endurance SSD drive and endurance-enhancing methods are needed.